Environmental Engineering Reference
In-Depth Information
O
2
-releases [H
2
O
2
+
light or enzymes (catalases/peroxidases)
→
O
2
+
2H
2
O
or other components] and their reused in H
2
O
2
generation in photosynthetic
organisms (O
2
+
Chl
+
H
+
+
h
υ
→
H
2
O
2
) can balance the O
2
level in the
environments in new photosynthetic reaction. But this does not occur in old pho-
tosynthetic reaction. (iv) Conversion of H
2
O
2
to O
2
occurs at a higher extent in
biological systems than in photoinduced decomposition processes (Moffett and
Zafiriou
1990
). For instance, the Cu(II) bathocuproinedisulfonic acid complex
(Cubc
2
) can convert H
2
O
2
to O
2
via the reaction (Eq.
4.1
) (Moffett et al.
1985
):
(4.1)
2CU
(
II
)
BC
2
+
H
2
O
2
→
2CU
(
I
)
BC
2
+
O
2
+
2H
+
Unconvincing evidence has been found for S
0
to S
4
transitions, and four suc-
cessive transitions are needed for H
2
O decomposition (Rappaport and Diner
2008
;
Kok et al.
1970
; Joliot and Kok
1975
; Krishtalik
1986
,
1990
).
(v) It is hypothesized that the O-O bond formation occurs when O5 in Mn
4
CaO
5
cluster provides one O atom via formation of hydroxide ion in the S
1
state (Umena
et al.
2011
; Saito et al.
2012
). A major issuer is then if it is possible to break down
O5 in the Mn
4
CaO
5
cluster. The problem is that, were it possible, probably the entire
PSII system would be broken down. Furthermore, each Mn atom in the Mn
4
CaO
5
cluster is in octrahedral form with six ligands, and it is also paramagnetic with 5
unpaired electrons in its outer
d
-orbitals (Mn
2
+
=
1
s
2
2
s
2
2
p
6
3
s
2
3
p
6
4
s
0
3
d
5
). The
result is that Mn could carry out strong H-bonding with other components. Such an
effect enables a second coordination sphere by D1-Asp 61, D1-His 337 and CP43-
Arg 357, in addition to the direct ligands. Therefore, these three residues might be
responsible for maintaining the oxygen-evolving activity (Umena et al.
2011
; Nixon
and Diner
1994
; Chu et al.
1995
; Hwang et al.
2007
; Service RJ, Hillier W, Debus
RJ
2010
). D1-Asp 61 is located at the entrance of a proposed proton exit channel
involving a chloride ion (Cl
−
) in Mn
4
CaO
5
(Umena et al.
2011
; Kawakami et al.
2009
; Guskov et al.
2009
; Murray and Barber
2007
; Ho and Styring
2008
). This res-
idue may facilitate proton exit from the Mn cluster. Proton releases from Mn
4
CaO
5
may play a key role in the formation of H
2
O
2
via O
2
•
−
•
.
(vi) Finally, the occurrence of about 1,300 water molecules in the PSII mono-
mer, located at the luminal and stromal sides (Umena et al.
2011
), could allow
the inclusion of a lot of dissolved O
2
molecules. They could add electrons after
they are released from Chl molecules upon excitation by light. Crystal structures
of PSI or PSII do not include any information about dissolved O
2
, and issue that
will need further studies to be clarified.
and HO
2
5 Factors Affecting the Photosynthesis of Organisms
Cyanobacteria carry out oxygenic photosynthesis using a photosynthetic system
similar to that observed in chloroplasts of higher plants. Therefore, cyanobacteria can
be used in model studies to understand the effects of various environmental factors